“The specified database is not a valid synchronization database” issue

Good Day Everyone,

Let me guess , if you are reading this article possible you may in the middle of a restore or migration issue and not able to restore User Profile Service Application .
This warning means you can not restore old sycronization database (default name is Sync DB) .This is a by design issue and you do not allowed to do it. that’s why you are getting fallowing warning.  For resolution just create a new Syncronization database. Don’t worry about the content in it because SharePoint will rebuild it once you configured User Profile Service Application correctly.